It isn't used by anyone besides me, but I wrote a property-testing library for Deno [1] that has a form of "sometimes" assertions (inspired by Antithesis) and uses "internal shrinking" (inspired by Hypothesis).

But it's still a "blind" fuzzer and it would be nice to write one that gets feedback from code coverage somehow. Instead, you have to run code coverage yourself and figure out how to change test data generation to improve it.

[1] https://jsr.io/@skybrian/repeat-test